This fish requires a specific weather transition, but other than that, it's a pretty straightforward straight catch. You will need Fish Eyes active, but its 4-minute buff means that you will never run a GP defecit.

You must mooch a High Quality http://xivdb.com/item/12780/sweetfish in order to catch this fish. Mind the weather! You can only catch this when Rain transitions into Showers, so it's a rare weather pattern. Otherwise, it's a fairly good catch rate and it shouldn't be much trouble.